The advent of portable computers has created intense demand for display devices which are lightweight, compact and power efficient. Since the space available for the display function of these devices precludes the use of a conventional cathode ray tube (CRT), there has been significant interest in efforts to provide satisfactory flat panel displays having comparable or even superior display characteristics, e.g., brightness, resolution, versatility in display, power consumption, etc. These efforts, while producing flat panel displays that are useful for some applications, have not produced a display that can compare to a conventional CRT.
Currently, liquid crystal displays are used almost universally for laptop and notebook computers. In comparison to a CRT, these displays provide poor contrast, only a limited range of viewing angles is possible, and, in color versions, they consume power at rates which are incompatible with extended battery operation. In addition, color screens tend to be far more costly than CRT's of equal screen size.
As a result of the drawbacks of liquid crystal display technology, field emission display technology has been receiving increasing attention by industry. Flat panel displays utilizing such technology employs a matrix-addressable array of pointed, thinfilm, cold field emission cathodes in combination with an anode comprising a phosphor-luminescent screen. The phenomenon of field emission was discovered in the 1950's, and extensive research by many individuals, such as Charles A. Spindt of SRI International, has improved the technology to the extent that its prospects for use in the manufacture of inexpensive, low-power, high-resolution, high-contrast, full-color flat displays appear to be promising.
Advances in field emission display technology are disclosed in U.S. Pat. No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing Such Structures," issued 28 Aug. 1973, to C. A. Spindt et al.; U.S. Pat. No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," issued 10 Jul. 1990 to Michel Borel et al.; U.S. Pat. No. 5,194,780, "Electron Source with Microtip Emissive Cathodes," issued 16 Mar. 1993 to Robert Meyer; and U.S. Pat. No. 5,225,820, "Microtip Trichromatic Fluorescent Screen," issued 6 Jul. 1993, to Jean-Frederic Clerc. These patents are incorporated by reference into the present application.
In flat panel displays of the field emission type, the electron emitting surface of the emitter plate and the opposed display face of the anode plate are spaced from one another at a relatively small distance over the extent of the display. This spacing, typically on the order of 200 .mu.meters (microns), limits the total volume of the cavity enclosed within an illustrative 10-inch diagonal display screen to less than 10 cm.sup.3.
In order for field emission displays to operate efficiently, it is necessary to maintain a good vacuum within the cavity of the display, typically on the order of 10.sup.-7 torr. The cavity is pumped out and degassed before assembly, but over time the pressure in the display builds up due to outgassing of the components inside the display and to the finite leak rate of the atmosphere into the cavity. As the pressure increases, the efficiency of the field emissions from the tip, and the phosphor luminescence decreases. Clearly, even the slightest leak rate or outgassing rate severely impacts a vacuum pressure level of 10.sup.-7 torr within the above-described minute cavity of the fiat panel display.
In evacuated display devices, getters are employed for adsorbing gases which are generated by components and gases which leak in from the atmosphere, so as to maintain a minimum pressure in the vacuum panel assembly. Since it is not currently known how to provide such a getter in any portion corresponding to the effective screen area, the getter is placed mostly in peripheral regions of the display device, frequently in the inactive regions between the front panel and the cathode outside of the screen area. As an example, in the apparatus disclosed in U.S. Pat. No. 5,063,323, "Field Emitter Structure Providing Passageways for Venting of Outgassed Materials from Active Electronic Area," issued 5 Nov. 1991, to R. T. Longo et al., outgassed materials liberated in spaces between pointed field emitter tips and an electrode structure are vented through passageways to a pump or gettering material provided in a separate space.
However, if the getter is positioned outside the effective screen area, this inactive external area must be dimensionally increased, which, as a consequence, substantially reduces the effective screen. There is also the disadvantage of diminution of the gas adsorption effect at the center of the screen, contributing to deterioration of the image quality. In one application known to the applicants, a field emission flat panel display includes a seal-off/pump-out tube on the back of the display, where a small piece of getter material, approximately two square inches, is placed. However, since new advances in field emission fiat panel display technology have made the seal-off tube unnecessary, this volume is no longer available for the placement of getter material. Since the FED has so little extra space inside the display cavity, there is no room for large pieces of conventional getter material. Without getter material to help maintain the vacuum, the useful lifetime of the display is shortened.
U.S. Pat. No. 5,223,766, "Image Display Device with Cathode Panel and Gas Absorbing Getters," issued 29 Jun. 1993, to A. Nakayama, discloses an image display having getter material in a space between a cathode panel and a back panel, and having holes in the cathode panel for adsorption of residual gases. In another embodiment of this patent, the cathode panel is supported from the back panel by a plurality of getters. In still another embodiment of the Nakayama patent, the gate electrodes are composed of a getter material.
U.S. Pat. No. 5,283,500, "Flat Panel Field Emission Display Apparatus," issued 1 Feb. 1994, to G. P. Kochanski, discloses active gettering devices comprising micropoints fabricated from one of the known getter metals. Evaporation of getter material occurs as a result of a potential which is selectively applied between the getter micropoint and the associated gate electrode. This approach, in which the evaporated getter metal is deposited on the anode, is deemed deleterious to the phosphor coating, and it is expected that the deposited getter will eventually result in significant deterioration of the display luminosity. It would also appear that the number of getter metal micropoints proposed by the patentee may not be adequate to provide proper gettering for the display.
In view of the above, it is clear that there exists a need for a flat panel display device having a substantial area of getter material, wherein the getter material is in close proximity to the display elements which are subject to outgassing, and in close proximity to those elements of the display which are adversely affected by increases in gas pressure. In addition, there exists a need for a getter material which is placed such it can be periodically reactivated within its operational configuration.